Boot the puter with the installation DVD. You will need to delete the current partition (which wipes it out and all the files) then create a new one in the space left behind, Windows 7 will format it for you by default.
System Manufacturer/Model Number DIY OS Windows 7 home premium x64 CPU AMD FX-4100 AM3+ 3.6GHz 12MB Black Edition Motherboard Asus M5A97 Pro Memory Crsair vengeance 12Gb DDR3 1600MHz CL9 Graphics Card Asus GTX 560 1GB Sound Card Realtek onboard Monitor(s) Displays Hanns G 1680x1050 native
PSU OCZ StealthXstream II 500W Hard Drives OCZ 128Gb Petrol ssd
2x500 Gb Samsung Internet Speed 8Mb or better
System Manufacturer/Model Number DIY OS Windows 7 home premium x64 CPU AMD FX-4100 AM3+ 3.6GHz 12MB Black Edition Motherboard Asus M5A97 Pro Memory Crsair vengeance 12Gb DDR3 1600MHz CL9 Graphics Card Asus GTX 560 1GB Sound Card Realtek onboard Monitor(s) Displays Hanns G 1680x1050 native
PSU OCZ StealthXstream II 500W Hard Drives OCZ 128Gb Petrol ssd
2x500 Gb Samsung Internet Speed 8Mb or better
It was mentioned already, but I'll toss it out again. Make sure you have a backup of anything important to you...because when you delete the partitions, the data is gone as well.